1. Field of the Invention
The present invention relates to a garnet crystal for growing a bismuth-substituted rare earth iron garnet single crystal film which is used in optical isolators and optical switches, and more particularly to a garnet crystal substrate which has a large lattice constant and allows solid solution of a large amount of bismuth in a bismuth-substituted rare earth iron garnet single crystal film.
2. Description of Related Art
A bismuth-substituted rare earth iron garnet single crystal (referred to as BIG hereinafter) is an excellent material that exhibits good transparency and large Faraday effect in near infrared region. Single crystal thick films having a thickness of several hundred microns have been practically used as a Faraday rotator in optical isolators and optical switches.
A BIG thick film having a thickness of several hundred microns is usually grown on a non-magnetic garnet substrate by a liquid phase epitaxial method. When growing such a thick BIG film, the BIG film and substrate must be very accurately aligned in lattice constant. A large difference in lattice constant results in defects in crystal structure or cracks in the substrate due to stress.
A garnet substrate for growing a BIG thick film includes an SGGG substrate made of (CaGd).sub.3 (MgZrGa).sub.5 O.sub.12 having a lattice constant of 1.2497 nm and an NGG substrate made of Nd.sub.3 Ga.sub.5 O.sub.12 having a lattice constant of 1.2509 nm. The Faraday effect and lattice constant of a BIG film increase with increasing amount of the bismuth-substitution and there-have been demands towards a garnet substrate having an even larger lattice constant. An article in "Journal of Solid State Chemistry VOL. 5(1972), P. 85ff." has reported on a garnet crystal having a large lattice constant. The article reports that garnet phase was observed when a mixture of rare-earth oxide and gallium oxide reacted under solid phase reaction.
In order to manufacture a substrate for growing a bismuth-substituted rare earth iron garnet single crystal film, a single crystal is required which has a diameter greater than one inch without cracks. However, there has been no report on such a single crystal yet.